**3.1 Materials**

*Current Topics in Biochemical Engineering*

magnetic stirrer with heating for the synthesis.

in bulk. Moreover, when the wavelength of the incident light is larger than the size of the nanoparticle exciting the plasmons at their natural resonance frequency, light is absorbed more strongly causing an increase in temperature. When the GNSs are synthesized with specific geometry and dimensions, their SPR changes causing their absorption to shift to the NIR region of the electromagnetic spectrum [3]. This shift offers a great potential for applications in the medical field because GNPs are bioinert [4], and the cytotoxicity of the silica has been widely studied [5]. Additionally, the wavelengths of the NIR spectrum are considered the optical window of the human body. As a result, while most biological soft tissues have low absorption of these wavelengths [6], GNSs absorb them causing them to increase their temperature. GNSs have been synthesized over different templates. Polystyrene cores claim to offer a narrower plasmon resonance absorption peak due to their higher reflective index [7]; iron oxide nanoparticles present a superparamagnetic template useful for magnetic resonance imaging [8]; silver nanoparticles have also been used as a mold for hollow gold nanoshells [9]. However, the functionalization of the polystyrene takes more time, reactants, and supervision which increases the chances of error as compared with the functionalization of the silica. Besides, the cytotoxicity of the silica nanoparticles makes them a good option for medical applications. Moreover, once the GNS is produced, the silica core can be diluted with hydrochloric acid to obtain hollow gold nanoparticles [10] that can be used for the controlled release of drugs [11] due to their capacity for encapsulating sensitive materials and their low thermal expansion coefficient. Therefore, providing a simpler and more efficient method of synthesis of GNS on silica templates provides a more promising variety of applications like for photothermal therapy [12], optical imaging [13], and drug release [2], as well as providing a near instantaneous in situ whole blood assay [14]. The synthesis of the GNSs has been extensively explored. Different methods, like reflux systems [15] or flow micro-reactors [16], can be used as well as procedures involving high temperatures [17]. But most of those methods last over 30 h [18]. In this chapter, we present a simple and effective method of preparation that shortens the time of the traditional procedures published before and uses only a

The reductions of the time were obtained by first modifying the Stöber method of synthesis of silica particles from 2 h to 30 min Samples were obtained at 30, 60, 90, and 120 min throughout the reaction to determine the minimum time of reaction needed. Also, the seeding process can be shortened from 2 h to 30 min. During the seeding process, where the silica is decorated with GNPs, a sample was obtained using only 30 min of resting time and compared with another sample obtained after the full 2 h of the resting time previously suggested. In both cases, SEM images were obtained showing that 30 min were sufficient to accomplish the synthesis of the silica as well as their seeding. In consequence, the total time of the process was reduced by 3 h.

The "Birth of Nanotechnology" was the title used by David Thompson [19] on his article acknowledging Michael Faraday's synthesis of gold nanoparticles in 1857. What Faraday called "Colloidal Ruby Gold" [20] was, in fact, a solution of dispersed GNPs so small that no microscope of that time was able to observe them. It wasn't until 1985 that Turkevich et al. [21] used an electron microscope to corroborate that Faraday's ruby gold was formed by GNPs with an average size of 6 ± 2 nm. Separately, in 1967 Werner Stöber et al. developed a method of synthesizing silica spheres in the micron size range [22] to be used especially in the medical field due to its known cytotoxicity, and in 1998 they were used by Naomi Halas et al. as the templates of GNS [23].

**90**

**2. Background**

Ethanol (100%), tetraethyl orthosilicate (TEOS) (98%), 3-aminopropyltriethoxysilane (APTES) (99%), trisodium citrate dihydrate, gold (III) chloride trihydrate (HAuCl4, 49%), formaldehyde (37%), and sodium borohydride (NaBH4, 98%) were purchased from Sigma-Aldrich. Potassium carbonate (K2CO3, 99%) and ammonium hydroxide (28%) were purchased from J.T. Baker. All the solutions were prepared with deionized water.
